Normally, if you make subsequent edits within a one hour period (configuration item ReplaceIfEditedAgainWithin), URCS will fold together your changes. This is often the "right thing to do", as it can reduce the visual clutter of diffs.

The "Force New Revision" checkbox is a way to force it to create a separate revision each time you save.

The TWiki.TWikiPreferences variable FORCENEWREVISIONCHECKBOX controls whether this is checked by default or not.

On a related note, you can force every save to be a new revision number by setting ReplaceIfEditedAgainWithin to 0.